3. Air Temperature

3.1. Temperature: A Measure
of Heat

Temperature
is an expression of the degree of hotness or coldness of a substance.

Temperature
can be measured using a thermometer. In this device, a liquid inside a glass
tube expands and contracts with temperature, and the position of the liquid
surface indicates the temperature. Another device measuring temperature is
called thermister, in this device, the electrical resistance changes with
temperature, and temperature is digitally measured.

There
are three types of units for temperature:

Celsius
(C ), international standard units. The reference points of this scale are the
sea-level freezing and boiling points of pure water, which are 0 and 100
degrees, respectively. The difference between freezing and boiling point is 100
degrees

Fahrenheit
(F): we all know that, the reference points on this scale are: 32 degree is
freezing point for pure water on sea-level. 212 degree is boiling point.

Kelvin
(K), mostly used as scientific research. The scale maintains a 100 degrees
between the boiling and freezing points of water but no negative values.

The
relationship between the three scales can be expressed

Degree Celsius = (degree Fahrenheit –32)/1.8

Degree
Celsius = degree Kelvin – 273

Since
air temperature can very with height, it is measured at a standard level –1.2 m
or 4 feet above ground at all weather station.

Although
some weather stations report temperatures hourly, most station only reports the
highest and lowest temperatures recorded during a 24-hour period.

Daily,
monthly, and yearly temperature statistics for a station are produced using the
daily maximum and minimum temperaure.

The
mean daily temperaure is defined as the average of the maximum and minimum
daily values.

The
monthly mean temperature for each station is determined by averaging the daily
means for each day in the calendar month.

The
annual mean temperature represents the average of the 12 monthly mean.

The
difference between the means of the warmest and coldest months is the annual
temperature range.

3.2 Daily temperature
pattern

Daily
temperature pattern is decided by daily net radiation pattern.

Figure
3.3 show average curves of daily insolation, net radiation, and air temperature
observed at lat. 40 to 45 N in the interior of North America.

Graph
(a) shows daily insolation. There are several features in this figure. (1) at
all time, the maximum insolation occurs at noon. (2) the maximum insolation
changes over time during the year.(3)
the time length of insolation also changes over time during a year. At the
equinox (middle) curve), insolation begins at about sunrise (6
am),
rises to a peak value at noon, and declines to zero at
sunset (6 pm). At the June solstice, insolation begins about two
hours earlier (4 am) and ends about two hours
later (8 pm). At the Decemeber solstice, insolation begins
about two hours later than the equinox and ends two hours earlier.

Graph
(b) shows net radiation for the surface. Net radiation is the difference
between incoming radiation and outgoing radiation. If the incoming radiation is
greater than outgoing radiation, then net radiation is positive or surplus,
otherwise, net radiation is negative or deficit. Compare graph (b) with (a), we
can see it is generally similar in shape. During the night, net radiation is
deficit and the deficit lasts till the sunrise and increasingly sharply and
reach maximum at noon and then decrease and to a
value of zero shortly before sunset.

Although
the three net radiation curves show same general daily pattern. They differ
greatly in magnitude. For the June solstice, the positive values are quite
large, and the area of surplus is much larger than the area of deficit. This is
means the net radiation for the entire day is positive. At the December, the total
deficit is larger than the surplus. This means the net radiation for the entire
day is negative.

Graph
(c ) shows the daily temperature pattern. The minimum daily temperature usually
occurs about half an hour after sunrise. Since net radiation has been negative
during the night, heat has flowed from the ground surface, and the ground has
cooled the surface air layer to its lowest temperature.As net radiation become positive, the surface
warms quickly and transfers heat to the air above. Air temperature rises
sharply in the morning hours and continues to rise after the noon peak of net radiation.

We
should expect the air temperature to rise as long as net radiation is positive.
However, another process begins in the early afternoon. Mixing of the lower air
by vertical currents distributes heat upward, offsetting the temperature rise.
Therefore, the temperature peak usually occurs in the mid-afternoon, from 2-4
pm
depending upon the local weather conditions

3.3.
Vertical temperature patterns

The
lower atmosphere can be divided into two layers:

TroposphereStratosphere

Height:0-13
km13-50
km

Temperature:
usually decrease with heights:Slightly
increase mainly because of

The
ozone absorb solar ultraviolet

6.4
C/1000 m (environmental lapse rate)

Water vapors, tiny dust (aerosol).very
little water vapor and dust because of strong wind but contains the ozone layer
which absorbs solar ultraviolet radiation and thus shields earthly life from
this intense, harmful form of energy.

Temperature
inversion: The most prominent exception to a normal lapse-rate condition is a temperature
inversion, a situation in which temperature in the troposphere increases,
rather than decreases, with increasing altitude.

They
occur typically on a long,cold winter night when a land surface rapidly emits
long-wave radiation into a clear, calm sky. The ground is soon colder than the
air above it and snow cools the air by conduction. In a relatively short time,
the lowest few hundred feet of troposphere become colder than the air above and
a temperature inversion is in effect.

High-mountain
environment

For
those of you who have climbed the high mountain, you might feel

(1)temperature decreases with
elevation;

(2)sunburn faster

(3)more difficult to breath;

(4)temperature at night drops
very quickly.

temperature
generally drop as you go up in elevation. In addition, you may feel sunburn is
a bit faster, also you may feel breathing is getting more difficult, and also,
you may feel at night, it is much cold than you expected. This is because, at
high elevation, the air is thin-that is , there are fewer gas molecules in a
unit volume of air. Of course, oxygen is fewer as well, that is why you feel a
bit difficult to breath. With fewer air molecues and dust particels to absorb
and scatter the sun’s light, so the sun’s ray will feel stronger. And also,
there is less carbon dioxide, so less radiation from the surface will be
trapped (called the greenhouse effect). As a result, the temperature will tend
to drop lower at night than you expect.

3.4. Annual temperature
patterns

As
the earth revolves around the sun, the tilt of earth’s axis causes an annual of
variation in insolation. This cycle produces an annual cycle of net radiation,
which, in turn, causes an annual cycle to occur in mean monthly air
temperature.

See
figure 3.12. it shows the yearly cycle of the net radiation and monthly mean
temperature at four sites with different latitudes.

Graph
(a) shows the annual cycle of the net radiation rate for four stations. At Manaus, located nearly on the
equator, the average net radiation rate is strongly positive in every month.
However, there are two minor peaks. These coincide approximately with the
equinoxes, when the sun is nearly straight overhead. A look at the temperature
graph of Manaus shows uniform air temperatures, averaging about 27
C (81 F) for the year. The annual temperature range, or difference between the
highest and lowest, is only 2.7 C (3 F). So there are no temperature seasons
near equator areas.

The
next station is Aswan, Egypt, located at Lat. 24. The
positive net radiation rate curve shows that a large radiation surplus exist
for every month. Furthermore, the net radiation rate curve has a much stronger
annual cycle, with values for June and July that are almost double those of
December and January. The annual range or difference is 17 C (or 30 F) and
summer (June, July and August) averaging 32 C (90 F).

Farther
north to the third station, Hamberg, Germany, located lat. 54. The net
radiation rate is positive for nine months and negative for three winter
months. Summer months reach a maximum of just 16 C (60 F), while winter months
reach a minimum of just about freezing (0 C or 32 F). The annual range is about
17 C, the same as at Aswan.

The
last one is Yakutsk, Siberia, Lat. 62 N. The net
radiation rate is negative for almost six months. During the winter , the air
temperatures drop to extremly low levels, between 35 –45 C (-30 and –50 F). In
summer, when daylight lasts most of a 24-hour day, the net radiation rates
rises to a strong peak, higher than the other three stations. The summer months
average temperature is about 13 C (55 F). the annual range is enormous-over 60
C (108 F).

Land-water
contrast

The
temperature pattern not only is affected by latitude but by the
location-maritime or continental. because oceans heat and cool more slowly than
continents.

Four
important thermal differences between land and water surfaces account for the
land-water contrast.

(1)Water is better transmitter
than rock or soils . So the radiation therefore warms a thick water surface
layer only slightly and a thin land surface layer more intensely.

(2)Water is slower to heat than
dry soil or rock. It takes about five times as much heat to raise the temperature
of the water one degree as it does to raise the temperature of the rock one
degree.

(3)Warm water can mix with cold
water

(4)Warm water can cool easily
by evaporation.

The
significance of these contrasts between land and water heating and cooling
rates is that both the hottest and coldest areas of earth are found in the
interiors of continents, distant from the influence of oceans. The places
located well inland and far from oceans generally experience a stronger daily
and annual temperature range.

Urban
and rural temperature contrasts

Temperature
in cities is usually higher than the surrounding rural regions. Because:

(1)difference
in surface materials. Surface in city is mainly covered with pavement or
structure, while rural areas are mainly covered with soil and vegetation.
Because the water can be evaporated from soil and vegetation into air in the
rural regions, so loss a considerable amount of latent heat, and as a result,
the temperature at the surface would cool down. While in the city, evaporation
takes place only in small proportion of areas such as lawns, street trees. So
during the daytime, temperature is higher than the rural area.at night, urban materials release stored heat
to the surface, also keeping temperatures warmer. Because the ability of
concrete, stone, and pavement to conduct and hold heat is better than the soil.

(2)
in the city, fuel consumption is much greater than the countryside. Summer, air
conditioner pumping heat out of building , releasing the heat to the air. In
winter, furnaces warm building. So interior heat is conducted to outside walls
and roofs, which radiate heat into the urban environment.

As
a result, the city itself is like a heat island.

3.5.
Global temperature pattern.

The
gross patterns of global temperature are controlled largely by four
factors-altitude, latitude, land-water contrasts, and ocean currents.

Altitude:
Temperature decreases with altitude

Latitude:
clearly the most evident feature of any world temperature map is the general
trend that temperature decreases with latitudes.

Land-Water
contrasts: the different heating and cooling characteristics of land and water
also affect the global temperature patterns.

Ocean
Currents: Warm ocean currents would increase the temperature.

Now
let’s see the world temperature map:

Isotherms:
lines drawn to connect locations having the same temperature.

Figure
3.18 shows the mean monthly temperature (C ) for January and July. Some
important features can be summarized as followings:

1.temperatures decreases from
the equator to poles.

2.Large landmasses located in
the subarctic and arctic zones develop centers of extremely low temperatures in
winter.

3.Temperatures in equatorial
regions change little from January to July.

4.Highlands are always colder
than surrounding lowlands.

5.Summer temperatures are
higher over continents than over the oceans, as shown by the poleward curvature
of the isotherms. Winter temperatures are lower over continents than over the
oceans; the isotherms bend equatorward over continents.

6.Enormous seasonal variation
in temperature occur in the interiors of high-latitude continents. At the other
extreme, the average temperature fluctuates only slightly from season to season
in the tropics, particularly over tropical oceans.